Spinach-Gruyere Gateau De Crepes

Ingredients

• 1/2 medium onion, cut into 1/4-inch dice
• 1 ounce (2 tablespoons) unsalted butter, plus more for sheet
• 2 tablespoons all-purpose flour
• 1 1/2 cups whole milk
• 1 1/4 cups grated Gruyere cheese (4 ounces)
• Pinch of freshly grated nutmeg
• Coarse salt
• freshly ground pepper
• 2 tablespoons extra-virgin olive oil
• 3 garlic cloves, minced (1 tablespoon)
• 5 ounces baby spinach (about 6 cups)
• Pinch of crushed red-pepper flakes
• 1 lemon, for squeezing
• 12 small Basic Crepes